What Causes Brownish, Short Periods With Nausea And Dizziness?

I've had some what of a period in the last two days, brownish to light red. I'm not feeling any cramps, but very neasua and dizziness. I thought I may be pregnant, but I took the test and it is negative. I have little simius of pregnancy, I am a little concern that it is still going on, I am pretty healthy and excercise well, I am always protected. I am just wondering what is wrong with my cycle?